1. Field of the Invention
The present invention relates to delay compensation for magnetic compressors in laser applications. In particular, the present invention relates to a method and system for providing temperature dependent delay compensation for magnetic compressors in excimer and/or molecular fluorine lasers.
2. Description of the Related Art
Magnetic compressors are widely used for applications that require short current pulses with high amplitude that exceed the specifications of commercially available semiconductor switches. For example, magnetic compressors can be found in excitation circuits for pulsed laser systems such as excimer or molecular fluorine lasers.
One disadvantage of magnetic pulse compression is that several factors influence the propagation delay through a magnetic compressor. For example, in an excitation circuit for an excimer laser, several stages of pulse compression can be used depending upon the compression factor as well as other requirements. A single compressor stage is typically made of a capacitor and a saturable inductivity that are comprised of a core made from a magnetic material and one or several windings.
The hold time, that is, the time needed to reach the saturation level and the low impedance state (referred to as switch through) is a function of the voltage across the core winding as well as other constraints such as the number of windings, the properties of the core material, and geometry, to name a few. This relationship can be seen from the following equation:∫Udt=constant  (1)
However, it is recognized herein that in application, the constant in the above relationship is not constant, but dependent upon temperature as the saturation flux of the core material is temperature dependent. Indeed, it can therefore be seen that the delay may be influenced by several parameters including the change in the operating voltage of the laser as well as heat generated from the dissipated energy.
In particular, when the voltage applied to the compressor stages is changed from laser pulse to laser pulse, or less frequently, to maintain the output energy of the laser constant, the dependency of the delay to the applied voltage can be observed as a non-linear relationship. For example, when the operating voltage of the laser is increased, the delay will decrease as can be seen from equation (1) above, since the integral shown above is understood to be constant with respect to this relationship between operating voltage and delay time.
Moreover, with each laser pulse, energy is dissipated in the core and the windings into heat such that, depending on the repetition rate and the effectiveness of the cooling, the temperature of the magnetic compressor is likely to increase. In addition, when the laser is operated in burst mode, the temperature is likely to decrease when a pause between bursts of laser pulses occurs. The change in the temperature in turn, affects the delay in the following manner. First, the saturation flux of the core material decreases with increasing temperature, and vice-versa, which, in turn, will drive the core earlier into saturation and the delay will decrease in the range of approximately 40 ns/° K. Additionally, the capacity of the ceramic compressor capacitors decreases by approximately 0.5%/° K, thus increasing the voltage according to the equation set forth below:E=(C/2)*U2=constant  (2)such thatU=constant*(1/C)1/2  (3)In the above, the main storage capacitor is taken to be a metal foil capacitor with a very small temperature coefficient (i.e., less than 0.01%/° K), and the stored energy is taken as constant at a fixed charging voltage and independent of the temperature.
It is recognized herein that the temperature dependence of the ceramic capacitors and/or saturable cores of a pulser circuit of a discharge circuit for an excimer and/or molecular fluorine laser can have a substantial influence on the delay due to the voltage changes by the capacitance modification. As can be seen from equations (1) and (3) above, thermally induced changes in capacitance can affect changes in charging voltage, and in turn, can affect changes in the delay. Moreover, the primary condenser may be a metal foil capacitor that does not indicate a substantial temperature dependence of the capacity, and thus, a loss of capacity of the ceramic capacitors leads to a rise in voltage, which then shortens the delay.
In general, a temperature dependent delay compensation circuit for a laser pulse circuit takes into account the temperature dependence of the delay due to the temperature fluctuations of the ceramic capacitors of the pulse compression stages of the pulser circuit. For instance, U.S. Pat. No. 6,016,325 discloses taking into account the temperature dependence of the saturation times of the saturable cores of the magnetic switch inductor elements of the circuit. The temperature of the cores is measured, and a delay is calculated based on the measured temperature, taking into account only the dependence of the saturation times of the saturable cores with temperature.
With so many factors affecting and potentially affecting the delay, measuring the temperature, taking into account the input voltage, burst information, etc., may still only allow an approximate delay compensation value to be estimated by calculation or using look-up tables such as temperature-delay compensation value tables, and/or including other input parameters such as high voltage, burst information, aging of components of the laser tube and/or discharge circuit. It is therefore desired to have a technique for stabilizing the delay between trigger pulse and electrical discharge for an excimer or molecular fluorine laser notwithstanding all the parameters such as temperature that fluctuate and influence the delay.